Refrigeration compressors used in refrigeration systems of the vapor compression type are commonly protected against conditions which may cause failure of the compressor, such as by a low oil pressure switch and a high head pressure switch. Air leaks into the closed refrigeration system can cause a catastrophic failure of a compressor, without a detection of a problem in time to save the compressor. Air, being non-condensable, increases head pressure, but the increased pressure may be below the setting of the head pressure sensor and still cause high internal compressor temperatures over a period of time. Over-temperature sensors have not been completely satisfactory in detecting and preventing compressor failure due to air leaks into the refrigeration system.